1. Field of the Invention
The invention relates generally to read/write devices for data carriers and more particularly to a read/write device of the type which is ramp-launched. It is applicable to magnetic read/write devices for magnetic peripherals of data-processing systems, in particular disc memories.
2. Description of the Prior Art
In present-day data processing systems, increasingly frequent use is being made of magnetic disc memories because of their storage capacity and the relatively short time taken by the magnetic read/write transducers to access an item of data contained anywhere on the discs from the time when the transducers receive an order to access this item from the said processing system.
Magnetic discs are known to carry data in concentric circular recording tracks whose radial widths are in the order of a few hundredths of a millimeter or less and which generally cover the major proportion of both faces of the discs. The discs are driven round at constant speed by an electric motor.
To enable data to be read from and written onto disc memories, magnetic transducers are arranged at a distance of a few .mu.m above both faces of the discs. The current practice usually is to associate a single platform provided with one or more transducers with each face of a disc. When it is desired to achieve a substantial reduction in the time taken by a transducer to access any item of data contained on the face in question, the platform is provided with a plurality of transducers. Typical magnetic platforms are illustrated and described in U.S. Pat. Nos. 3,723,665 and 3,846,847 and copending application Ser. No. 882,592 of Lazzari et al, filed Mar. 2, 1978, now U.S. Pat. No. 4,198,667, and assigned to the assignee of the present invention.
The transducers used are generally magnetic transducers which contain a magnetic circuit having an air gap and a winding arranged round it. The air-gap enables data to be read and written. Generally, but not exclusively, platforms incorporating one or more transducers are in the shape of a relatively shallow rectangular parallelepiped of which the major face, termed the lower face, which is adjacent the discs, contains the air gaps of the transducers, and of which the opposite major face, termed the upper face, contains the electrical terminals and wires which enable the windings of the transducers to be connected to the electrical circuits of the disc memory which are associated with them.
When the transducers of a platform are not performing the operation of reading or writing data from or on the corresponding face of the disc, the platform occupies a so-called "rest" position in relation to the disc which is different from the so-called read or write position.
The platforms are provided with a fitting which, as a whole, forms a device which will be referred to herein as a read and/or write head.
There are two main types of read/write heads. One is known as a "ramp-launched head" or "ramp-landed head", and the other is known as an "unstick from contact head".
"Ramp-landed heads" have long been known. They generally comprise: a platform incorporating at least one transducer, a member to carry the platform at one of its ends which takes the form of a substantially rectangular rigid strip, a rigid arm detachably secured to a carriage intended to move the platform above the disc, a loading spring formed by a substantially rectangular flexible strip, one side of which is fixed to the arm and the other side of which is fixed to the member carying the platform at the end of the said member opposite from that carrying the platform, and a means for controlling the position of the platform in relation to the data carrier. The latter means comprises a guide member made up of a level surface and a ramp and a pin which, when it cooperates with the level surface, holds the platform in its rest position and which, when it cooperates with the ramp, brings the platform to its reading/writing position. The level surface forms an extension, generally in the same plane, of the rigid strip carrying the platform. The ramp forms an extension of the level surface in an upwardly inclined plane, that is to say it extends away from the face of the disc.
Given that in this structure the rigid strip which is to carry the platform and to incorporate the position controlling means forms an extension of the loading spring, the platform necessarily carries a heavy loading. A typical value for this loading is in the order of 300 g. It is desirable, however, that the loading on the platform should be light.
Assuming that the disc rotates, typically, at 60 revolutions per second, and that, for reasons inherent in its manufacture, the surface of the disc contains unevenesses which pass in front of the platform at a frequency of 600 Hz, the platform needs to be capable of accepting severe acceleration in order to be able to follow the undulating surface of the disc. Since acceleration is the quotient of force divided by mass, it is possible to achieve high acceleration either by increasing the force or by reducing the mass. However, since the platform flies above the disc at the time of reading or writing as a result of the lifting effect of a cushion of air which exists between the lower face of the platform and the disc, force is a datum which cannot be altered. The problem is therefore to reduce the mass of the head. This being 300 g at the present time for typical structures, it would be desirable to reduce it to 15 g, that is to say to divide it by 20. Unfortunately, it is impossible to produce a landing head having so low a loading while still retaining a conventional structure as described above.
As another drawback of heavy loading, mention should be made of the sensitivity of such heads to the unwanted dust particles that the face of the disc may be carrying. In effect, the head is not sufficiently compliant to react to the presence of large particles which infiltrate between one of its air gaps and the disc. Such particles may act as magnetic shunts and adversely effect the quality of the detected signal and may also cause mechanical damage to the adjoining faces of the disc and the platform.
To overcome the disadvantages of ramp-landed heads, another type of head has been designed termed the "unstick from contact head". Such heads are illustrated and described in copending applications of Michel Plotto, Ser. Nos. 958,839 and 958,840, filed Nov. 8, 1978, now U.S. Pat. Nos. 4,225,891 and 4,212,044, respectively, and assigned to the assignee of the present invention. These new heads are being more and more widely used by reason of their low loading. The platforms for these heads have a lower major face termed the flight surface. The flight surface comprises a first part which is in the form of skids which rest on the disc when the latter is stationary and a second, bevelled part, which is inclined to the skids and which is situated at the front of the platform, the front being that part of the flight surface which, when the face of the magnetic disc associated with the platform is passing in front of the flight surface, is the first in time to have the data on this face of the disc pass before it. The air gaps of the transducers belonging to the platform are situated at the rear of the flight surface.
When it is desired to move the platform from its rest position to its flying position above the face of the disc which is associated with it, it is merely necessary to set the disc in rotation. After a fairly short time, the platform rises due to the lifting effect created by the bevel. When the disc reaches its nominal speed of rotation, the platform is in its flying position with the skids inclined relative to the face of the disc, which provides the platform with sufficient lift for it to fly stably above the disc. The platform is thus enabled to take off by the lift provided by the skids.
Platforms of this kind are carried by a weak leaf-spring, which, in the read or write position, also acts as an arm to guide the platform above the disc and to suspend it.
Despite the advantage which these platforms have by virtue of their being lightly loaded, their use is restricted by the following two principal drawbacks. Firstly, the platform for such a head has an unstick time and, although this time is only of the order of a few milliseconds, it does mean that after a number of take-offs the flight surface of the platform and the corresponding face of the disc deteriorate as a result of the wear on them caused by the mechanical friction between them. Secondly, replacing a disc poses far more problems than it does with ramp-launched heads. In effect, unstick from contact platforms have to be moved away from the disc and then, when the disc has been removed, they have to be held in a position where they cannot be damaged and finally, they have to be moved out of the way again while the new disc is being inserted and adjustments have to be made to ensure that they fly at the correct height.
In contradistinction, ramp-landed heads do not have these advantages inasmuch as the platform is fairly well spaced from the disc in the rest position thereby enabling easy disc replacement. When the disc is in place adjustment is relatively easy.